1. Field of the Invention
The present invention generally relates to a negative photosensitive composition sensitive to radiation and a method for forming a resist pattern by means thereof. More particularly, it relates to a negative photosensitive composition useful as a negative photoresist suitable especially for production of semiconductor integrated circuits.
2. Discussion of Background
A negative photoresist in which cyclized rubber and a bisazide compound as a crosslinking agent are used, is well known as a resist useful for photolithography. However, with such a negative photoresist, since swelling takes place during development by the action of an organic solvent used as a developer, the resolution of the resist results in 3 .mu.m at best. Therefore, such a negative photoresist is not suitable for the production of a highly integrated device. Further, the organic solvent used for the development is problematic from the aspects of environmental pollution and health or in view of inflammability.
Whereas, a positive photoresist comprising an alkali-soluble novolak resin and a compound containing a naphthoquinone diazide moiety as a photo-acid-generating material, is well known as a photoresist to which an alkali developer can apply. However, such a positive photoresist has a substantial absorption at 300 nm or less and thus has a drawback that when exposure is conducted at a short wavelength, the pattern profile will be very poor. Therefore, exposure has to be conducted at a wavelength of at least about 350 nm, whereby the resolution is limited, and it is not useful enough for lithography of less than a half micron level.
Although a conventional lithography employing exposure at 350 nm or longer has not sufficient resolutoin, a high level of resolution can be obtained even by employing a light with a wavelength of at least 350 mm such as i-line (365 nm) of a mercury lamp, if exposure is conducted by a phase shifting method using an improved reticle. However, it is well known that for the phase shifting method, a negative resist is more advantageous than the positive resist.
For example, Japanese Unexamined Patent Publication No. 164045/1987 discloses a negative photoresist composed of a combination of an acid curable resin and a photo-acid-generating material. Specifically, such a photoresist contains methoxymethylated melamine (Cymel.RTM. 303, trade name, manufactured by Mitsui Cyanamid) as a crosslinking agent. Japanese Unexamined Patent Publication No. 170165/1990 discloses a radiation sensitive composition which comprises a compound capable of generating an acid when irradiated with a radiation and an alkali-soluble phenol resin containing methoxymethyl groups, methylol groups or both of them, particularly an alkali-soluble polymer having at least structural units of the following formula: ##STR1## wherein each of n and m is an integer of 1 or 2, and R is hydrogen or methyl.
However, it is still desired to develop a negative photoresist having various properties including resolution further improved.